Differentiate both sides w.r.t. xxx; for parametric x=f(t),y=g(t)x = f(t), y = g(t)x=f(t),y=g(t): dydx=g′(t)f′(t)\dfrac{dy}{dx} = \dfrac{g'(t)}{f'(t)}dxdy=f′(t)g′(t).